1. Field of the Invention
The present invention relates to a semiconductor laser driving circuit for driving a semiconductor laser utilized in optical recording and to an optical head incorporating a semiconductor laser and a driving circuit thereof.
2. Description of the Related Art
The amount of information handled under a variety of circumstances has been dramatically increasing. It is thus desired to increase the storage capacity and the data transfer speed of the recording system that performs writing and reading of such information. It is especially an important technical issue to make the writing speed of the large-capacity optical recording system using an optical storage medium such as an optical disk as high as that of magnetic disk drives. Developments have been thus increasingly made in optical storage media adaptable to the high writing speed.
In general the optical recording system incorporates an optical head disposed to face an optical storage medium. The optical head applies light to the medium and data is thereby optically recorded on the medium. A semiconductor laser is generally used as a light source of the optical head. The semiconductor laser is driven by a semiconductor laser driving circuit so that the laser emits pulsed light when data is written. The driving circuit incorporates a switching element that generates a driving signal having rectangular waves that controls the timing of emission of the laser. The driving signal is then given to the laser.
To make the writing speed of the optical recording system higher, it is required to make the output of the semiconductor laser greater and to make the pulse width of the light emitted from the laser smaller. To reduce the pulse width it is important to make the transmission line through which the driving signal is sent as short as possible so that degradation in the waveform of the driving signal on the line is prevented. The driving circuit therefore incorporates an integrated circuit (also referred to as IC in the following description) dedicated to driving the semiconductor laser, and the driving circuit is mounted on the optical head in many cases.
In the operation of the above-mentioned IC for driving the semiconductor laser, ripple components are superposed on the supply voltage of the driving IC as the switching element is operated. The ripple components superposed on the supply voltage generate unfavorable ripple components, too, on the driving signals. In general, a capacitor having a large capacity is therefore connected between the ground line and the power line connected to the IC for driving, so that the ripple components superposed on the supply voltage of the IC are reduced. In prior art the location in which this capacitor is disposed is not particularly considered.
A conventional driving circuit so far has the ability to generate a driving signal for having the semiconductor laser emit pulsed light having a desired pulse width. For example, the minimum pulse width of pulsed light is approximately 29 ns when data is written on a compact disk rewritable (CD-RW) at a quadruple speed. The conventional driving circuit is capable of generating a driving signal for having the semiconductor laser emit pulsed light having a width of this order without any particular problem.
To make the width of pulsed light shorter, however, it is necessary that the driving circuit generate a driving signal having rectangular waves of shorter width, for further increasing writing speed.
The rise time of a wave of a driving signal typically used, such as a rectangular wave signal generated by a transistor-transistor logic (TTL), is of the order of 1 ns. For an actual electronic circuit, however, there exist impedance mismatches at the input and output, and a floating impedance on the transmission lines. As a result, rounding is produced at the rise of a wave of the driving signal. This rounding causes distortion of waves of pulsed light emitted from the semiconductor laser.
Such distortion of waves of pulsed light will not cause problems of the operation of the optical recording system if the pulse width of pulsed light is sufficiently great. If the pulse width is reduced, however, the proportion of distorted portions of the waves of the entire pulsed light is made greater. As thus described, when data is written on an optical storage medium through the use of pulsed light having a great proportion of distorted portions of the waves, it is possible that energy having a sufficient power will not be supplied to the medium. As a result, data will not be correctly written on the medium and the possibility of occurrence of reading error will be increased.
As thus described, the conventional semiconductor laser driving circuit has the problem that it is not capable of generating a good driving signal, particularly when the pulse width of pulsed light emitted from the semiconductor laser is reduced.